1. Field of the Invention
The present invention relates to a semiconductor substrate having a layer of gettering sink material formed thereon, a method of manufacturing the semiconductor substrate and a semiconductor device using the semiconductor substrate.
2. Description of the Background Art
A semiconductor device is fabricated by combination of the steps of film formation, washing, thermal treatment, photolithography and the like. Heavy metals such as Fe, Ni, Cr, Cu, Zn and the like are inevitably mixed in, for example, a washing solution for use in the step of washing and resist for use in the step of photolithography. Therefore, these heavy metals often enter a semiconductor device.
One example of the problems that occur when heavy metals enter a semiconductor device is that, when heavy metals enter a PN junction, the region of the junction acquires a crystal defect to cause current leakage.
Among steps for manufacturing a semiconductor device, the step of removing heavy metals which entered the semiconductor device is referred to as gettering. There are various kinds of gettering. The following description will be given to gettering carried out by formation of a poly-crystalline silicon film on the rear surface, having no elements formed thereon, of the surface of a silicon single-crystalline wafer.
When the poly-crystalline silicon film is formed on the rear surface of the silicon single-crystalline wafer, gettering is carried out on the following two theories.
(Theory 1)
FIG. 23 is an enlarged cross section of a portion of a silicon substrate 2. Silicon substrate 2 has a poly-crystalline silicon film 5 formed on a rear surface 9 of a silicon single-crystalline wafer 1. A number of grains 11 gather to form poly-crystalline silicon film 5. A boundary of respective grains 11 is called a grain boundary 13.
A natural oxide film 3 is formed between rear surface 9 and poly-crystalline silicon film 5. Natural oxide film 3 is formed inevitably when poly-crystalline silicon film 5 is formed on rear surface 9.
An MOS (Metal-Oxide-Semiconductor) field effect transistor 17 is formed on a main surface 7 of silicon single-crystalline wafer 1. Heavy metals entering by any means are shown by reference numeral 15.
Heavy metals 15 have a characteristic of gathering at a crystal defect when energy is imparted thereto. Grain boundaries 13 serve to be crystal defects. When energy is imparted to heavy metals 15 in the step of thermal treatment, heavy metals 15 gather at grain boundaries 13. As a result, heavy metals which entered a semiconductor device are removed.
(Theory 2)
FIG. 24 is an enlarged cross section of a portion of silicon substrate 2. Description will not be repeated to the same elements as those in FIG. 23 by labeling the same reference numerals. During the step of thermal treatment, silicon in poly-crystalline silicon film 5 enters silicon single-crystalline wafer 1 through natural oxide film 3. As a result, a crystal defect 10 is generated in silicon single-crystalline wafer 1. Heavy metals 15 to which energy is imparted in the step of thermal treatment gather at crystal defect 10.
FIG. 25 is a cross section of a conventional silicon substrate having a poly-crystalline silicon film formed on the rear surface. Silicon substrate 2 includes silicon single-crystalline wafer 1 having natural oxide film 3 formed on rear surface 9 thereof and poly-crystalline silicon film 5 formed on natural oxide film 3. The thickness of natural oxide film 3 indicated at A is approximately 20 .ANG.. The thickness of poly-crystalline silicon film 5 indicated at B is approximately 1 .mu.m. A main surface of silicon single-crystalline wafer 1 is indicated at reference numeral 7, on which semiconductor elements are formed. Such a semiconductor substrate is disclosed in, for example, U.S. Pat. No. 4,053,335.
Brief description will be given to a method of manufacturing a silicon substrate shown in FIG. 25 with reference to FIG. 26. An ingot of silicon single crystal is prepared. This ingot is sliced. Edges of this sliced silicon single-crystalline wafer are chamfered. Chamfering rounds off edge portions of the wafer, thereby preventing edge portions of the wafer from chipping.
The surface of the chamfered wafer is smoothed by lapping and etching. In other words, the surface of the chamfered wafer is roughly ground by lapping. A lapping material, fragments of silicon, a damaged layer and the like are removed from the lapped wafer by etching. The etching solution adhering to the wafer is washed away in water.
Poly-crystalline silicon film 5 is formed on the whole surface of the etched wafer. Natural oxide film 3 is formed naturally on silicon single-crystalline wafer 1 after completion of etching and before formation of a poly-crystalline silicon film.
Main surface 7 of the wafer having a poly-crystalline silicon film formed thereon is then mirror polished. This state is shown in FIG. 25.
However, it was not possible for silicon substrate 2 of a conventional type and having poly-crystalline silicon film 5 formed on rear face 9 of silicon single-crystalline wafer 1 to have satisfactory gettering effect.